Gas guage inaccurate. on 2005 Kia Sorento

Guage goes to 'full' when filled up. then after using 5 to 7 gal. drops off a 1/4 each time car is started until it shows empty, and stays there. Is float separate from sending unit? Does tank have to be dropped? Does new gas additive cause this damage?

Are you saying this all started after adding gas additive? Float is part of the sender assembly. Found this online:

The nature of the gauge is that it should behave with a linear taper between full and empty. If it works when full but then malfunctions as the level drops, the variable resistor in the sending unit which is housed along with the fuel pump in the gas tank is almost certainly at fault. The sending unit will have to be replaced to fix the problem. Expect this to be an expensive endeavor

I have the EXACT same probelm with my 2004 Sorento...Driving us crazy. Took to dealership 3 times while it was under warranty and they couldn't figure it out! I read earlier on another website to check the sender unit wires to make sure they have good connections and not corroded. Also check the grounding wire. I'm going to check all this tonight....hope it works!